Crash writing and reading back class instance address to void*

[Remi Thebault via Digitalmars-d-learn]

Hi

Starting to use GtkD TreeModel, I write an instance of an 
abstract class to TreeIter.userData.

When reading back the void pointer and casting to my abstract 
class leads to crash when instance is used (Task is the abstract 
class):


        int fillIter(TreeIter iter, Task t)
        {
            if (!t || !iter) return 0;

            iter.stamp = stamp_;

            writeln("writing ", cast(void*)&t);
            iter.userData = cast(void*)&t;

            return 1;
        }


        Task taskFromIter(TreeIter iter)
        {
            if (!iter || iter.stamp != stamp_) return null;

            writeln("reading ", iter.userData);
            writeln(cast(Task)iter.userData);
            return cast(Task)iter.userData;
        }

the code prints
writing 18FC98
reading 18FC98

and crashes at the 2nd writeln call in taskFromIter function with 
message object.Error@(0): Access violation

The instance is referenced somewhere else in the model, so it 
should not get garbage collected. should I check this anyway?

Any idea?

thanks
Rémi